This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Osteoarthritis Initiative (OAI) is a multicenter observational cohort study designed to identify biomarkers for the development and/or progression of osteoarthritis of the knee. Men and women aged 45-79 years of age are targeted for enrollment. Interested parties are interviewed over the telephone by a trained interviewer. If deemed eligible, a screening visit is scheduled. If eligible based on the radiographs obtained at the screening visit, an enrollment visit is scheduled within 6 weeks. Each enrolled participant is expected to complete four annual follow-up clinical visits which are comparable to the enrollment visit. Each clinical center (there are 4, including Baltimore) is expected to recruit 1,250 subjects;for Baltimore, this number will be split between the University of Maryland Baltimore site and the Johns Hopkins Bayview site.